Human nature is conditioned by heredity, physical environment, and cuture.
There are many different genetic causes of mental retardation, thus indicating that many genes are necessary for"normal" mental function. Recent research is directed
toward the role of genetic factors in human behavior. Some implications of these observations are suggested.

[Error in original article]
structure of hemoglobin. Many of these techniques are
applicable to the investigation of human behavior (5).
A standard method of approach is to select a group

Man's Uniqueness

Man is different from animals in many ways but not
in being free from the laws of heredity. Rife (13)
lists the unique features of man as a larger brain which
enables him to dominate all other animals, language
ability so that he can communicate ideas with his fellow men, the use of tools, invention of the making and
use of fire, and the ability to be educated and pass on
the culture of one generation to the next.

Bases of Man's Nature

What are the bases for these characteristics? Rife
(13, p. 115) says, "Human beings are the products of
biological inheritance, culture, and physical environment." Man's distinctive features are made possible by
his genetic endowment, and certain genetic changes can
drastically alter an individual's behavior pattern. One
should not disregard the other two influences, however.
Someone was asked, "Which is most important in making man, heredity, environment or culture?" and he
replied, "Which is the most important leg of a threelegged stool?" It is the purpose of this paper, nevertheless, to deal primarily with the topic of inheritance.

Men are different from one another. They are hereditarily different. Their equality is before the law and
their Creator, but not in their hereditary material.
Hence, to study the nature of inheritance, we observe
the variations in traits that appear among men and try
to learn from such differences what are the hereditary
factors.

The bearers of heredity are the genes, which act as
a "code" to direct the formation of many different enzymes. These enzymes act upon the chemicals supplied
through the embryonic environment, and the net result
is a human body with certain characteristics or traits.
We should realize that one gene may affect several characters, while several genes may have an effect on one character. Snyder (19, p. 8) states, "This point of view is
especially important when it is realized that the
environmental events intervening between gene actions
and finished characters in man may range from such
overt occurrences as trmuna and infection to the most
subtle embryological, immunological, and psychological
phenomena."

Much of our behavior is controlled by brain physiology. To the extent that the brain is affected in de
velopment or function by genes, to that extent behavior
is affected by heredity. To have a brain at all one must
have "good" genes (and a suitable embryonic environment). If brain defect may be the result of "bad"
genes, the converse is also true, that normal anatomy
and physiology of the nervous system require "good"
genes. We may learn about normal function by study
ing malfunction.

Methods of Studying Human Inheritance

The student of human genetics encounters certain
problems. He cannot control mating nor standardize the
environment There is a long interval between birth and
reproduction, so that the "generation time" is about as
long as the research life of the investigator. The large
number of chromosomes (46 per cell) makes the locating of genes on specific chromosomes difficult.

Fortunately, important advances have been made possible recently by new techniques (4). Computer programs now permit complex calculations upon large
masses of data. Human chromosomes can be identified
readily in white blood cells from tissue culture preparations. Biochemical studies have led to the identification
of genetic enzyme defects and genetic differences in the

Based partly upon a paper presented at the 1962 convention of The American Scientific Affiliation, Bethel College,
St. Paul, Minn.

**Dr. Anderson is Assistant Director of the Dight Institute
for Human Genetics, University of Minnesota; Dr. Mixter is
Professor of Zoology at Wheaton College, Wheaton, IL.

of persons with a specific problem, such as schizophrenia. The frequency of the same condition among parents, brothers and sisters, and children can then be
compared with what would be expected with different
patterns of inheritance
(1).
The effects of family environment would often produce a different pattern, and
this possibility can be tested as well. If the disorder being studied is a rare one, and if the affected children
often have parents who are blood relatives, this information by itself is good evidence for a simple
genetic cause.

The study of twins can also be revealing. Twins are
called identical if they have the same sex and are similar
in many traits, such as blood groups and eye color. They
are labeled fraternal if they show differences. Identical
twins are assumed to have the same inheritance from
one egg and one sperm, but fraternal twins are the products of two sets of germ cells.

By comparing the occurrence of characteristics in
identical twins with the occurrence in fraternal twins,
we get a clue to traits which are inherited. If a trait occurs more often in both identical twins than in both
fraternal twins, the trait is considered to have an hereditary basis. The text by Stern
(20)
has a very clear description of the logic behind such reasoning.

If a husband and wife have a child born without a
brain, the likelihood of another brainless child is about
three percent. However, it turns out that among twins
"discordant identical twin pairs are almost the rule, that
is, one affected and the other not. Even so, this does
not dispose of a familial tendency; it merely shows that
environmental peculiarity of some sort is necessary to
evoke the reaction to which both twins must be susceptible"
(10, p. 75).

A number of different twin studies have been carried
out for schizophrenia, a mental disorder which occurs
in about one percent of the population. Certain problems in the analysis of twin data have led to criticism
of this approach, but Rosenthal (14) concluded that
the total weight of the evidence from all studies still
strongly favors the hypothesis of a genetic contribution
to the etiology of schizophrenia. Among identical
twins both members are affected in two-thirds of the
pairs, while only one-sixth of fraternal twin pairs show
the abnormality in both members.

Some may suspect that a similar environment has
caused the appearance of mental illness in both of
identical twins. But Stem
(20, p. 580)
reviews Kallmann's account of identical twins reared apart who both
became schizophrenic. One worked in a factory and had
an illegitimate child at the age of fifteen; the other was
a housekeeper for a good family. The fact that both
developed the disease in spite of different backgrounds
suggests a constitutional weakness. Shields
(17)
studied
a number of identical twins and found that in some
personality traits those pairs reared apart were more
alike than those reared together. It is possible that
twins reared together develop somewhat different habit patterns in an attempt to establish a personal identity.

Mental Retardation

There are many different factors which can lead to
mental retardation. Some of these may be classed as environmental-irradiation of the developing fetus, birth
injury, or infection, for example.

In addition there are at least fifty different types of
mental retardation which result from genetic causes
(2,
16). A wide range of biochemical defects are included, involving the body's use of amino acids, carbohydrates, lipids, or hormones. Some types cannot be
described biochemically as yet, but the family history
shows them to follow simple laws of inheritance.

We are ignorant of the existence of a gene until we
detect a mutation of it. Then we know that all along
there was a "normal" gene in place of the new "abnormal" one. Thus, if hereditary changes are identified
which lead to mental retardation, we are justified in
saying that many genes are required for normal mental
function.

An exciting break-through in recent years was the
discovery of an explanation for mongolism, also called
Down's syndrome. The size of the problem can be seen
from the fact that among four million babies born in
the United States each year about
8,000
will show the
signs of this disorder
(12).
In 1959, French scientists Lejeune, Turpin, and Gautier identified the cause as an
extra chromosome (number
21)
in the cells of mongoloids. Normal babies have 46 chromosomes and mongoloids have 47.

What causes the presence of the extra chromosome?
Apparently some process which accumulates with age
causes non-disjunction, that is, puts an extra chromosome into a germ cell. The chance of having a child
with Down's syndrome increases sharply with the age
of the mother, reaching about two percent for mothers
over forty.

Other mental abnormalities result from unusual
numbers of chromosomes
(8).
Severe retardation accompanied by multiple physical defects results from an
extra chromosome in the group numbered
13-15
or an
extra chromosome
18.
Men who have extra X chromosomes are retarded and have undeveloped sex organs.

Normal Intelligence

Stem (20, p. 590) summarized the studies which
have been made on the response of normal individuals
to intelligence tests. Identical twins (reared together)
differed by only
3.1
points, as compared with an
8.5
point difference between non-identicals. Even identical
twins reared apart have only a 6 point difference. A
large part of the latter difference occurred in four pairs
of twins who also had the most marked difference in
amount of schooling. Stern concluded that the twin
studies show: (a) that the I. Q. score is modified by
differences in environment, and (b) that identical twins,
whether reared together or apart, show greater similarity in I. Q. than non-identical twins reared in the
same home.

Differences in Behavior

A number of studies have been made on the gross
differences in behavior seen in the psychotic disorders
such as schizophrenia and manic-depressive psychosis
(12, 18).
When all are taken into account, there is
good evidence for genetic differences in ability to respond to environmental circumstances. Paul Meehl, in a
presidential address before the American Psychological
Association (9, p.
837),
stated his personal conviction
that psychologists will be able "to make a unique contribution in the near future, using psychological techniques to establish that schizophrenia, while its content is learned, is fundamentally a neurological disease
of genetic origin."

Attempts to find a single biochemical change in all
cases of schizophrenia have been unsuccessful. But there
is growing evidence of unusual biochemical findings in
a few psychotic persons
(11).
A different enzyme defect has been observed in persons who appear normal
except for mild speech problems (6). We can expect to
find a series of other biochemical causes for some cases
of behavior disorder.

Another type of behavior problem is criminality.
Stern
(20, p. 605)
concluded a discussion on this
point: "The facts on criminality in twins actually show
only that identity in genes plus the close similarity in
social experiences, at least in early childhood, are more
identical twins into prison than two
On the basis of present data, one
cannot exclude the possibility that the higher concordance for criminality of identical twins is mainly, or even
exclusively, the result of their more similar social experiences; nor can one exclude the opposite possibility
that their higher concordance is mainly the result of
their identical genotypes . . . If the present evidence
is insufficient to indicate reliably that criminals are
'born,' it does point to a genetic component for the type
of crime committed, if a crime is committed at all."

Some data are available on behavior differences
among the general population. In his study of identical twins brought up apart and together, Shields
(17,
p. 156)
found a significant resemblance between twins
in both groups with respect to "intelligence, extraversion, neuroticism, and a variety of personal characteristics, ranging from voice and mannerisms to smoking
habits." Gottesman
(7)
tested twins with two personality inventory tests and found an appreciable hereditary effect on certain of the rating scales. When the data
were compared with the profiles for various psychoneurotic syndromes, the importance of genetic factors
was highest for schizoid/schizophrenic reaction and
anxiety reaction. Conversion reaction or hysteria rated
the lowest. Vandenberg
(21, p. 235)
used a different
battery of psychological tests and found that "hereditary factors play a role in many areas of human skilled
performances, often in spite of the fact that these skills
are highly practiced."

The experimental analysis of behavior differences is
receiving a good deal of attention in the emerging field of behavior genetics
(3, 5).
It is clear that gentic factors are involved in behavior differences between strains
of fruit flies, of mice, or of dogs. It is equally clear that
behavior patterns may be conditioned and modified by
early experience. In young animals, for example, there
appear to be critical periods during which the basic social relationships can be established most readily (15).
Such findings will give us new ideas and methods which
can be adapted for the study of human behavior.

Some Implications for the Future

From this discussion, one can see the importance of
biological factors in our understanding of the nature of
man. The abnormal development of the nervous system
or a change in blood chemicals can drastically alter behavior. Furthermore, there may be considerable inborn
variability from person to person.

In a number of different aspects of behavior one's
genetic endowment probably establishes a "repertoire"
of mechanisms for reacting. Early environment is then
crucial in permitting or restricting the actual expression
of these ways of responding.

a) Such thoughts should sharpen the problem of
human responsibility and sin. This does not mean at all
that the power of sin is reduced. But it does suggest
that oar diagnosis of what constitutes sin in another
person may not be as accurate as we assume. Is it not
possible that for certain behavior problems a medical
treatment is more helpful than prayer?

b) Further research into die complex interactions between heredity and environment should be encouraged.
"The need for more and more knowledge concerning
die genetic bases for human behavior provides the
greatest challenge of all to the human geneticist" (13,
p. 241).

c) Variability in human behavior is desirable. Certain severe disorders are clearly a problem, but aside
from these there is no set of traits that is the best.
Diversity should be accepted and encouraged.

d) Much work is needed to find ways of correcting
the effects of gross behavior disorders. It is possible that
different methods of education are required for different
groups of children in order to help each child make the
best use of his potential abilities.

REFERENCES

1. Anderson, V. Elving, "Statistical Studies of Probands and
Their Relatives,"
Annals of the New York Academy of
Sciences, 91:781-796, 1961.

2. Anderson, V. Elving, "Genetics in Mental Retardation," in
Review of Research in Mental Retardation,
Chicago: University of Chicago, In press.

17. Shields, James, Monozygotic Twins Brought up Apart and
Brought up Together: An Investigation into the Genetic
and Environmental Causes of Variation in Personality,
London: Oxford University Press, 1962.